Form-pressed products (also called form-bent products or shaped/curved plywood products) typically consist of from 3 to 50 layers of veneer glued together with an adhesive. The gluing process is performed in a mould to give the final product the desired shape. The most commonly used adhesive for this application is urea formaldehyde type adhesive (UF) as it gives excellent performance as well as being inexpensive. UF adhesives are, however, not the only adhesive type used for form pressing. In cases where increased moisture resistance is required, Melamine Urea Formaldehyde (MUF) adhesives are used. In cases where formaldehyde free solutions are required PVAc or Emulsion Polymer Isocyanate (EPI) adhesives are used. PVAc adhesives can only be used for very simple construction without much tension due to the thermoplastic nature of PVAc adhesives. For more challenging constructions, EPI adhesives may be used. The use of EPI adhesives offers challenges such as handling of the isocyanate hardener, limited pot life, limited assembly time, relatively long pressing times, potential of creep of the glue line (due to the thermoplastic nature of the adhesive) and high cost.
UF adhesives are based on formaldehyde and will, depending on the adhesive formulation, give more or less emission of formaldehyde from the final product. With the increased focus on formaldehyde emission and increasingly stricter restrictions of the emissions this is posing a greater and greater problem. This has been accentuated by some customer's decision of using formaldehyde free adhesives in all products where this is possible as well as the suggestion of very strict Californian regulations regarding formaldehyde emission from glued products.
Over the years the UF technology has been improved to minimize the amount of formaldehyde that is emitted from the glued products. For chipboards this has resulted in a reduction of formaldehyde emission from a level typically about 100 mg/100 g before 1970, to the present level of about 5 mg/100 g (according to the EP test perforator method EN120). There is however a limitation to how much can be done with a UF system as the bond quality is getting reduced as the emission is lowered. This is a very big problem for form bent products where the final product usually contains a high number of glue lines (from 2 to 49) and a large amount of glue. Each glue line must emit close to no formaldehyde to give a product with the required low emission. It is an object of the invention to provide a solution to this problem.